A snowmachine clutch actually consists of two separate
clutches connected together by a rubber belt.

The primary clutch is connected to the engine's
crankshaft

The secondary clutch is mounted on the end
of the jackshaft (which connects to the drive shaft via a chain and gears).

The primary purpose of the clutch is to smoothly
transmit power from the engine to the jackshaft and to remove the connection
when the engine is idling so that the machine is not always rolling.

This type of system is also referred to as a continuously
variable transmission. It is called this because as the engine
speed increases the final drive ratio increases. That is, the difference
between the engine speed and track speed decreases. It is equivalent
to an automatic transmission on a car with an infinite number of gears
that you never felt shift. Let's take a look at how this "infinite
gearing" process works:

At idle the primary clutch just spins and does not
"grab" onto the belt. As engine RPM (revolutions per minute) increases
to engagement speed (the RPM where the snowmachine starts moving), the
primary clutch begins to pull together and start squeezing the belt.

Now the belt is turning. This makes the secondary
clutch turn, which causes the track to turn and the snowmachine to move
forward.

As engine speed increases above engagement, the
primary clutch squeezes together some more and pushes the belt so that
it moves to a larger radius on the primary. Because the two clutches
rotate about fixed points, the belt gets pulled into the secondary, spreading
it farther apart and moving the belt to a smaller radius.

To make the above movie play, double click it.

Note that the primary clutch (on the left) never
stops spinning, but the secondary (right) does stop spinning at idle speeds.

In case you are curious, the engine rpm got
up to about 7000 rpm and the track speed got up to about 60 mph or more
during this clip. I supported the track with a stand and ran the throttle
while an observer ran the camera.

It's at this point I should make a comment about
not trying this at home. As the manufacturers state, you should never
operate the engine with the clutch guard removed or the hood up, and especially
not both at the same time!

Image taken from http://www.snowest.com/

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Primary Clutch
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